How to Use P150 Sanding Discs in Paint Prep – Grit Sequence Guide for Professionals

How to Use P150 Sanding Discs in Paint Prep – Grit Sequence Guide for Professionals

The Ultimate Grit Sequence Guide: How to Use P150 Sanding Discs in Automotive Paint Prep

In the world of automotive surface preparation, choosing the right sanding sequence is as critical as the paint itself. Whether you're working on collision repair, panel refinishing, or full vehicle repainting, your grit progression will determine the quality of your final finish.

Among the most widely used abrasives in the industry is the P150 sanding disc — a mid-grade grit ideal for leveling primer, blending repair edges, and preparing surfaces for paint adhesion. But when and how to use P150 in the correct sequence often confuses even experienced technicians.

This guide provides a complete breakdown of grit sequencing in professional paint prep, highlighting the proper role of P150 discs, common mistakes, and field-tested application techniques.

Why Grit Sequence Matters in Paint Prep

Before applying any paint system — basecoat, clearcoat, or topcoat — the substrate must be correctly abraded. Skipping grits or choosing the wrong progression can lead to:

  • Visible sanding scratches under the clearcoat

  • Poor paint adhesion

  • Orange peel effect

  • Premature paint failure

A consistent, step-by-step grit sequence ensures that each stage of sanding removes the scratches from the previous stage, reducing swirl marks and creating a uniform surface profile for the next process.

Where P150 Sanding Discs Fit in Your Workflow

P150 is a versatile grit that's used during intermediate surface leveling. It plays a transitional role between aggressive shaping (e.g., P80–P120) and final finish preparation (P240–P320).

Professionals often choose P150 for:

  • Feather-edging repair zones

  • Smoothing putty or primer surfaces

  • Refining high spots on body filler

  • Transitioning from filler to OEM paint

 

Common Mistakes When Using P150 Sanding Discs

1. Skipping Grit Steps

Many technicians jump from P80 or P100 straight to P180, skipping the vital mid-step provided by P150. This leads to deeper sanding scratches being partially hidden — but still visible after painting.

Fix: Use P150 as a bridging grit after P120 and before P180. It minimizes scratch depth and improves primer uniformity.

2. Applying Too Much Pressure

Overloading the disc with pressure generates heat, wears out the abrasive faster, and can distort the panel — especially on lightweight aluminum or plastic.

Fix: Let the disc cut on its own. Use controlled orbital action with moderate hand pressure, especially when working on curved or thin areas.

3. Using a Worn-Out Disc

When discs lose their grit, they tend to smear or polish the surface instead of abrading it. This causes uneven scratch patterns and poor primer adhesion.

Fix: Replace discs at appropriate intervals, and store them in dry, clean environments to prevent contamination.

4. Sanding Without Dust Extraction

Modern sanding discs, particularly multi-hole hook & loop types, are designed to be used with a vacuum system. Operating without extraction leads to clogging (loading), reduced cutting power, and inconsistent results.

Fix: Always use dust extraction compatible back-up pads with P150 discs. Clean the surface and tool between each step.

5. Inappropriate Pad or Machine Choice

Using a rotary sander without a soft interface pad can create swirls or dig into the surface. Similarly, applying a disc to a hard Velcro base without a cushion layer risks over-cutting.

Fix: Use the correct machine (DA or orbital), matched with a soft interface pad, especially when feathering around edges or transitions.

Advanced Tip: Using P150 Before Primer

Although some techs use P180 before primer, P150 often provides better mechanical adhesion, especially on high-build primers. It leaves a coarser but uniform scratch pattern that primers can "grab," improving hold and reducing sinkage.

If using P150, ensure the primer system is designed for medium-cut surfaces, and follow with a P240 or P320 pass before basecoat application.

Conclusion: Control the Sequence, Control the Result

P150 sanding discs are a workhorse in auto body surface prep. But their power lies in when and how they're used within the full sanding workflow.

By following proper grit sequencing and avoiding common application errors, you'll not only achieve a better finish — but save time, reduce rework, and increase the longevity of your paint system.